1. Field of the Invention
This invention generally relates to systems configured to provide illumination of a specimen or to inspect a specimen. Certain embodiments relate to a system that is configured to multiply the repetition rate of pulses of light generated by a light source such that the pulses of light can be directed to a specimen as quasi-continuous-wave illumination.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
Defects in reticles are also a potential source of yield reduction in integrated circuit manufacturing. Therefore, inspection of a reticle is a critical step in the reticle manufacturing process and after release of the reticle to production. As minimum pattern sizes shrink and integrated circuits are designed with higher device densities, reticle defects that were once tolerable may no longer be acceptable. For example, a single reticle defect may be repeated in each die in stepper systems and may kill every die in single-die reduction reticles. In addition, VLSI and ULSI-level integrated circuit manufacturing require substantially defect-free and dimensionally perfect reticles due to the critical dimension (CD) budget of such manufacturing.
Generally, sensitivity of an inspection system is related to optical resolution for techniques that use optical microscopes to examine wafers and reticles for defects. Therefore, one obvious way to improve the detection of relatively small defects is to increase the resolution of an optical inspection system. One way to increase the resolution of an optical inspection system is to decrease the wavelength at which the system operates. For instance, resolution is defined as: Resolution=λ/n(NA), where λ is the wavelength, n is the index of refraction, and NA is the numerical aperture of the optical system at the object. Therefore, resolution increases as wavelength decreases.
The minimum wavelength of current inspection and defect review tools is in the range of 248 to 266 nm. For example, current generations of reticle inspection tools use laser illumination at a wavelength of about 257 nm generated by an intra-cavity doubled argon (Ar)-ion laser. Next generation tools need to operate at a sub-200 nm wavelength and will require multiple stages of nonlinear wavelength up-conversion. With continuous-wave (cw) lasers, the multiple stages of up-conversion will require multiple resonant build-up cavities in addition to the ion laser, resulting in a light source that will have a high cost of ownership (expensive to purchase and maintain) and poor reliability. Therefore, the lack of inexpensive cw light sources for wavelengths below 200 nm make such light sources impractical for next generation inspection and defect review tools.
Mode-locked lasers are an attractive alternative for the sub-200 nm generation of tools because these lasers are based on solid-state laser technology. In addition, their high peak power permits efficient single-pass nonlinear wavelength up-conversion without the use of external resonant build-up cavities. A mode-locked laser may replace a cw laser without extensive inspection tool re-architecture if it operates at a sufficiently high pulse repetition rate. A calculation based on the requirements for speckle-contrast reduction indicate that a pulse repetition rate of greater than about 200 MHz is required for mode-locked lasers to be a viable option. However, mode-locked lasers are much more efficient when operated at pulse rates of about 80 MHz to about 90 MHz.
Accordingly, it would be advantageous to develop systems configured to provide illumination of a specimen such as a reticle or a wafer for applications such as inspection that include a simple and efficient optical configuration for quadrupling the repetition rate of pulses emitted by a light source such as a mode-locked laser.